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W. H. Baldridge, C. N. Morgan, J. E. Nason; An Inhibitor of Tyrosinase Delays Light-Induced Cone Movements in Goldfish Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1272.
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Retinomotor movements (RMMs) in fish can be modulated by exogenous dopamine but there is some uncertainty about the source of endogenous dopamine that could mediate light-induced RMMs (Douglas et al., 1992; Ball, Baldridge & Fernback, 1993). It has been demonstrated in zebrafish that adaptation to bright light requires tyrosinase, an enzyme involved in the synthesis of melanin, but possibly also L-DOPA, in the retinal pigmented epithelium (Page-McCaw et al., 2004). L-DOPA synthesized by tyrosinase could be converted into dopamine and influence light-induced RMMs. To test this hypothesis we investigated the effect of a blocker of tyrosinase, phenylthiourea (PTU), on light-induced contraction of cones in the goldfish retina.
Adult goldfish (Carassius auratus) were exposed to 0.2 mM (30 mg/L) PTU, dissolved in the tank water, for a period of 48 hrs prior to study and maintained on a 12 hr light/12 hr dark cycle. Control fish were treated identically but in tank water without PTU. At the mid-point of the light cycle, the fish were placed in the dark for 2 hrs and then exposed to 0, 3.5, 4.5, 5.5, 6.5 or 7.5 min of illumination (100 Cd/m²) prior to sacrifice. Fish sacrificed after 2 hrs of darkness and no subsequent illumination were manipulated using an infrared imaging system. Eyes were removed and fixed in 4% paraformaldehyde containing 0.5% glutaraldehyde in phosphate buffered saline (PBS). After fixation the eyes were washed in PBS and pigment bleached by treatment with 3% hydrogen peroxide. Sections were cut using a modified McIlwain chopper and viewed using DIC microscopy. Cone position was measured as the distance between the outer limiting membrane (OLM) and the cone outer segment within a 2.5 mm radius surrounding the optic nerve.
In control retinas, a light-induced change in cone position (from a dark-adapted cone length of 80 ± 4 µm, mean ± SD, n=6 fish) was first detectable after 4.5 min illumination (48 ± 11 µm, n=4) and reached the maximal light-induced position after 6.5 min illumination (18 ± 4 µm, n=4). In retinas from fish treated with PTU, the first detectable change in cone position was delayed relative to control, becoming evident after 5.5 min illumination (51 ± 8 µm, n=6) and did not reach the fully light-adapted position (21 ± 2 µm, n=6) until 7.5 min of light.
Light-induced cone movements in goldfish were delayed but not blocked by treatment with an inhibitor of tyrosinase. These results add to the evidence that tyrosinase activity may provide a source of dopamine in the retina.
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